This invention relates to an improved process for preparing small spheres of silica having many applications, such as for catalyst supports and in high pressure liquid chromatography. Such spheres are small, but very uniformly sized.
Stober, in U.S. Pat. No. 3,634,558 disclosed a method for making monodisperse silica spheres. An alcoholic aqueous ammonia solution was prepared and tetraalkyl silicates (i.e., tetraalkoxysilanes) were added with agitation. The effect of varying alcohols and alkylsilicates was reported. Stober noted that two different size particles were produced and to avoid this he reported that he relied on maintaining a uniform temperature of 22.degree. C. and homogeneous suspension by gentle agitation. Generally, the patentee found that particles above 1 .mu.m were difficult to produce with ethanol and ethyl esters. Making larger particles required the use of esters of higher alcohols. Of major importance was the effect of attaching radioactive tracers to the spheres.
Particles in the range of 0.1 to 1.0 .mu.m were prepared by Yuasa et al., as discussed in U.S. Pat. No. 4,567,030. The patentees incorporated metals from Groups I, II, III, and IV of the Periodic Table with the hydrolyzable silicon compounds such as tetraalkyl silicates to form particles containing both silica and the selected metals as their oxides. They state that if the content of the metal oxide is 0.01 to 15 mol percent a true sphere with a uniform particle size is obtained. The patentees also observe that the amount of water affects the ability to produce spherical shape. Also, they state that it is ". - - - indispensable that a mixed solution of both the starting compounds should be prepared in advance." In their comparative example, the patentees demonstrate that spheres are not formed if silica and titanium precursors are added separately. Thus, Yuasa et al. teach that both silica and another metal oxide should be present in order to prepare uniform spherical particles, although only in the relatively small size range of 0.1-1.0 .mu.m. In discussing the conditions affecting the particle size, the patentees observe that increasing the concentration of base (ammonia is preferred) increases the size of the particles and that increasing the water concentration also increases the particle size and further, that organic solvents having a greater number of carbon atoms produce larger particle sizes. Yuasa et al. did not discuss the effect of temperature on the particle sizes and judging from their examples they maintained the reaction mixture at 20.degree. C. while the particles were being formed and grown.
A non-porous spherical particle of only silica was made by a different method discussed in U.S. Pat. No. 4,775,520 to Unger et al. Their particles were said to have mean particle diameters of 0.05 to 10 .mu.m, although no particles larger than 3.1 .mu.m were actually exemplified. A two-stage process was employed which was similar to one method of Yuasa et al. in that continuous addition of a tetraalkoxysilane was used to increase the size of the particles. The patentees defined their process as adding the silica precursor solution to a preformed sol of seed particles at a rate at which substantially no additional particles were formed, that is, the number of particles initially added determined the number of spheres produced. It was characteristic of the particles formed by Unger et al. that they were non-porous and highly uniform. They assumed that pores in the original seed particles are sealed by the secondary growth and that new pores do not form. Unger et al. recommend using reaction temperatures between 35.degree. and 75.degree. C., preferably between 40.degree. and 65.degree. C. They state that around room temperature larger particles are formed but that a wider range of particle sizes was produced. Their examples indicate that the reaction mixture was controlled at 40.degree. C.
Shimizu et al. in U.S. Pat. No. 4,842,837 disclose a process for making fine silica spheres less than 0.1 .mu.m used as a polish for semi conductor wafers. Hydrolysis of an alkoxysilane was carried out above 30.degree. C. and the patentees indicated that lower temperatures were not desirable since larger spheres were formed. Their examples indicate that a constant temperature was maintained throughout the reaction.
Barder et al. in U.S. Pat. No. 4,983,369, which is incorporated by reference, disclose a method of producing microspheres of silica which involves the use of precursor solutions which are initially of such proportions that two-phase mixtures would form. This method produces highly uniform spherical particles. Barder et al. suggested that the reaction be carried out at 10.degree. C. to 50.degree. C. but did not indicate that control of the temperature was important to their results.
The previous workers in the field have suggested, but have not shown, that large, monodisperse spherical particles could be made. The present inventor has discovered an improved method of making uniform spheres which is capable of producing larger particles than have heretofore been available.